1,2,3,9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazole-1-yl)methyl]-4H-carbazol-4-one or its salt is a useful compound as an anti-vomiting agent due to its selective action on 5-HT3 receptor, which is represented by the following chemical structure:

The compound of formula 1 may be prepared by various methods, for example, as disclosed in GB Pat. No. 2,153,821, EP Pat. No. 219,929, EP Pat. No. 221,629, KR Pat. No. 0217466, KR Pat. No. 0216422, KR Pat. No. 0377578, and KR Pat. Pub. No. 2002-0039223 (corresponding to EP Pub. No. 1207160).
The process disclosed in GB Pat. No. 2,153,821 may be summarized as following reaction scheme 1:

However, the above processes have the problems that the processes are completed through so many steps, which take so long time for carrying out the reactions of each step, and the yield of final product is very low.
The process disclosed in EP Pat. No. 219,929 may be summarized as following reaction scheme 2:

In the above reaction, an anhydrous condition is required in order to introduce a dimethylamino group. Further, the process is completed through so many steps and employs an expensive agent. Accordingly, the process disclosed in EP Pat. No. 219,929 has difficulties to be applied to an industrial-scale mass production.
The process disclosed in EP Pat. No. 221,629 may be summarized as following reaction scheme 3:

However, the above process also has difficulties to be applied to an industrial-scale mass production in that the yield of final product is very low and the process employs very expensive catalyst.
The process disclosed in KR Pat. No. 0217466 may be summarized as following reaction scheme 4:

However, the above process has the problems that the process employs excess acetic acid in order to prepare the compound of formula 2, which makes it difficult to carry out a work-up process after completion of reaction, and an additional isolation process is necessary in order to remove impurities.
KR Pat. No. 0217466 discloses a process for preparing the compound of formula 1, which comprises reacting the compound of formula 2 with an acid salt of 2-methyl imidazole. However, the reaction described in KR Pat. No. 0217466 is carried out at a high temperature, i.e. above 130° C., which is unfavorable for industrial-scale mass production.
The process disclosed in KR Pat. No. 0377578 and KR Pat. Pub. No. 2002-0039223 (corresponding EP 1207160 A1) may be summarized as following reaction scheme 5:

The above process has the problem that the process requires an additional reaction step in order to prepare the starting material of formula 16, which have to be isolated by high vacuum distillation. Accordingly, the above process has difficulties to be applied to an industrial-scale mass production.